Laurent Heux

Mercerization of primary wall cellulose and its implication for the conversion of Cellulose I -› Cellulose II

The mercerization of homogenized primary wall cellulose extracted fromsugar beet pulp was investigated by transmission electron microscopy (TEM),X-ray diffraction together with 13C CP-MAS NMR, and FT-IR spectroscopy.For samples resulting from acid extraction, mercerization began at 9% NaOH, whereasfor samples purified by alkaline treatment, the mercerization started at 10%NaOH. The change in morphology when going from cellulose I to cellulose II wasspectacular, as all the microfibrillar cellulose morphology disappeared duringthe treatment. This change in morphology was very drastic as soon as the NaOHconcentrations were increased beyond 8 and 9% for the acid and alkalinepreparedsamples, respectively. On the other hand, the conversion was found to be moreprogressive in terms of increasing NaOH concentration when the transformationwas analyzed by X-ray diffraction or spectroscopy. Our observations of themercerization of isolated cellulose microfibrils are consistent with theconceptof cellulose microfibrils made of parallel chains in cellulose I and crystalsofcellulose II consisting of antiparallel chains.

Structural aspects in ultrathin cellulose microfibrils followed by 13C CP-MAS NMR

13C CP-MAS NMR was used to study the ultrastructural aspects of ultrathin microfibrils extracted from sugar beet pulp. Depending on the stocking condition, the cellulose microfibrils contained different amounts of the polymorphs Iα and Iβ. Structural changes were followed after a purification treatment. It was found that the crystallinity, as measured by solid-state NMR, increases as the purification proceeds. This is primarily due to the removal of non-cellulosic polysaccharides during the different treatments. This was confirmed by neutral sugar analysis. A crystallite size of 4 nm is derived from the NMR results which is in good agreement with the TEM observations. To avoid any underestimate of the microfibrils size, most of the hemicelluloses tightly bound to the cellulose microfibrils has to be removed.

NMR measurements of scalar-coupling distributions in disordered solids

The measurement of scalar (J) couplings by solid-state NMR is a field of great interest, since this interaction is a rich source of local structural information, complementary to dipolar and chemical shift interactions. Here, we first demonstrate that J-coupling distributions exist and can be observed in disordered solids, as illustrated with the observation of a pair-specific distribution of (2)J((31)P-N-(31)P) couplings in a bis-phosphino amine, and we investigate the potential effects of such distributions on the measurement of average J-coupling constants. Second, we show that the measurement of two-dimensional (2D) distributions of J-couplings provides a much richer probe of local structural disorder than one-dimensional distributions, and we introduce new methods that provide different (selective or non-selective) ways of measuring 2D J distributions in a wide range of disordered systems. These methods are finally applied to a slightly disordered polymorphic sample of fully (13)C-enriched cellulose, and then to the bis-phosphino amine sample, from which 2D (2)J(PP)-coupling distributions are clearly identified and interpreted.

Iα → Iβ transition of cellulose under ultrasonic radiation

Aqueous suspensions of dispersed Glaucocystis cellulose microfibrils were sonicated at 4xa0°C for 3xa0h, using 24xa0kHz ultrasonic waves. This treatment induced a variety of ultrastructural defects, as the microfibrils became not only shortened, but also presented substantial damage materialized by kinks and subfibrillation. Upon analysis by X-ray diffraction and 13C solid-state NMR spectroscopy, it was found that the initial sample that contained 90xa0% of cellulose Iα allomorph became, to a large extent, unexpectedly converted into the Iβ phase, while the loss of crystallinity was only moderate during the sonication treatment.

Heterogeneous reaction between cellulose and alkyl ketene dimer under solvent-free conditions

Four native celluloses, i.e. hardwood bleached kraft pulp (HBKP), cotton linters, bacterial cellulose and microcrystalline cellulose, were heterogeneously reacted with alkyl ketene dimer (AKD) at 100xa0°C for 24xa0h in the presence of a small amount of 1-methylimidazole used as an esterification promoter under solvent-free conditions. AKD molecules were reacted with cellulose hydroxyl groups to form β-ketoester linkages by the above conditions. Each reaction product was separated into chloroform-soluble and -insoluble fractions, and weight ratios and degrees of substitution (DS) of the two fractions were measured. The weight ratios of the CHCl3-insoluble fractions varied from 30 to 95xa0% of the products, and the DS values from 0.03 to 1.6, depending on the starting cellulose samples. X-ray diffraction and solid-state 13C-NMR analyses of the CHCl3-insoluble fraction with DS 0.75 in the AKD-treated HBKP indicated that β-ketoester substituents were introduced to hydroxyl groups on the cellulose microfibril surfaces. This CHCl3-insoluble fraction had thermoplastic properties, and was convertible to translucent or transparent films by hot pressing at 160–180xa0°C.

Crystal transition between hydrate and anhydrous (1→3)-β-D-xylan from Penicillus dumetosus.

The crystal structures of hydrated and anhydrous (1→3)-β-d-xylan and the crystal transition between them were investigated. Highly crystalline samples of (1→3)-β-d-xylan were prepared from a siphoneous green alga, Penicillus dumetosus. The crystal structure was analyzed using synchrotron X-ray diffraction and solid-state (13)C NMR spectroscopy. The hydrated form was converted to the anhydrous form with contraction in both the a-axis and c-axis directions, and the crystallinity decreased considerably at the same time. The crystal transition between hydrated and anhydrous (1→3)-β-d-xylan was monitored using synchrotron X-ray diffraction under controlled relative humidity. The transition took place at certain transition points, and was accompanied by a large hysteresis. From the difference in the weight and unit-cell volume at the transition points, the number of water molecules in the hydrated form was estimated as one per xylose residue.